1. Field of the Invention
The invention relates to microwave filters that are compatible with microwave monolithic integrated circuit (MMIC) technology, and more particular to a microwave channelized bandpass filter having two channels.
2. Description of Background Art
There is an urgent need for small, light-weight, low-cost microwave filters that are compatible with microwave monolithic integrated circuit (MMIC) technology, yet offer low insertion loss and high selectivity. Such filters are needed particularly for next-generation ultra-compact multi-function systems for defense applications. There is also a need for such filters in frequency synthesizers and in commercial mobile communication systems.
The concept of the microwave channelized filter, as described in the present inventor's U.S. Pat. No. 5,339,057 and in C. Rauscher, "Microwave Channelized Active Filters . . . ," IEEE Transactions on Microwave Theory and Techniques, pp. 122-123, January 1996, both expressly incorporated by reference, marked a major breakthrough with regard to the realization of miniature, highly selective microwave filters. The breakthrough is based on an active-filter concept that uses parallel-connected, frequency-selective network branches to accomplish filter selectivity through constructive and destructive interference among branch signal components, thereby circumventing limitations related to circuit stability and noise that have rendered earlier microwave active-filter approaches impractical. The feed-forward architectures of these filters resemble those of analog transversal filters, and exhibit performance and operational advantages similar to those of transversal filters. However, the channelized filters use frequency-selective feed-forward signal paths (instead of the frequency-independent ones used in transversal solutions) which, in turn, reduces the required number of signal paths or branches to two or three, down from the fifty to one hundred branches needed in the case of analog transversal filters. The space savings are dramatic, without compromise of performance.
Channelized filters with low-pass, high-pass, and band-reject responses all require a minimum of two feed-forward branches. On the other hand, channelized filters with bandpass responses, which encompass the majority of applications, have to date required a minimum of three branches. Adding the third branch causes the actual filter size to increase by far more than the simple three-to-two ratio which is suggested by the respective numbers of branches in the two types of circuits. This is due to the relatively large circuit area needed to realize three-way signal splitters and combiners, when compared to the much smaller and simpler two-way splitters and combiners used in low-pass, high-pass, and band-reject situations.